One method which has been used for treating arrhythmic conditions within a patient's heart involves the use of antiarrhythmic drugs such as sodium and calcium channel blockers or drugs which reduce the Beta-adrenergic activity. Another method includes surgically sectioning the origin of the signals causing the arrhythmia or a conducting pathway for such signals. Another frequently used method to terminate an arrhythmia is to destroy the heart tissue at the site from which the signals causing the arrhythmia originate or tissue in a pathway through which such signals pass. The methods used for destroying heart tissue include applying laser, radio frequency (RF) energy or microwave energy to the patient's endocardium at or close to the site from within the patient's left or right ventricle in order to destory heart tissue involved with the arrhythmia and thereby terminate the irregular heartbeat. The average arrhythmogenic site consists of a projected area of about 1.4 cm.sup.2 of endocardial tissue, and a re-entrant site might be much larger. Unfortunately, presently used RF ablation techniques produce lesions about 0.5 cm.sup.2 in diameter, so a number of lesions usually must be generated in order to effectively ablate an area of interest sufficiently to terminate the arrhythmia. If the site is not accurately mapped or if there is difficulty in accurately placing and/or holding the distal tip of the ablation device, good tissue surrounding the site which is neither the cause of nor involved with the arrhythmia will be unnecessarily destroyed.
Prior ablation methods typically used elongated intravascular devices with distal portions disposed within a chamber of the patient's heart which have ablating means such as an RF emitting electrode or a laser delivering optical fiber held in contact with the desired region of the patient's endocardium to be destroyed. While this procedure is now widely practiced, precise positioning of the distal ablation portion of the intravascular device at the desired location where the tissue causing or involved with the arrhythmia is to be destroyed remains a formidable problem.
Another method for terminating arrhythmia which has been experimentally used is the chemical ablation of the region of the patient's heart from which the irregular electrical activity originates, such as described by Brugada et al. in "Transcoronary Chemical Ablation of Ventricular Tachycardia", Circulation (1989); 2:475-482. In this procedure, iced saline is first directed through a coronary artery which is believed to deliver blood to the ectopic foci or arrhythmogenic site. If the arrhythmia is temporarily terminated by the iced saline indicating that the arterial vessel feeds the origination site, then an aqueous solution of ethanol is delivered through the arterial passageway to lyse the tissue at the origination site and permanently terminate the electrical activity of such tissue. The alcohol solution quickly occludes the arterial passageway at the location into which it is introduced causing a myocardial infarct in the arrhythmogenic region of the patient's heart. Unfortunately, the ethanol solution frequently refluxes to other regions of the patient's heart, making this procedure very difficult to control to a small region of the patient's myocardium. The region of resulting tissue damage can be much larger than necessary to merely terminate the arrhythmia. Due to the extensive risk of extraneous damage to the patient's myocardium from chemical ablation, this procedure has not been used except In limited experimental trials. See also Haines et al. in "Intracoronary Ethanol Ablation In Swine", J. of Cardiovascular Electroohvsiology, (1994); 5, No. 1, 41-49 and de Paola et al. in "Transcoroanry Chemical Ablation of Ventricular Tachycardia in Chronic Chagasic Myocarditils"J. of the Ameican College of Cardiology (1994); 20, No. 2, 480-482.
What has been needed and heretofore unavailable is a method and system for lysing tissue at a site which causes or is involved with the arrhythmia but which does not destroy an excessive amount of uninvolved tissue. The present invention satisfies these and other needs.